1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter, “HST”).
2. Related Art
Conventionally, there is a well-known HST having its neutral position expanded.
For example, there is a conventional transaxle apparatus equipped with a power transmission system including a conventional HST 8 for enabling a vehicle to travel forwardly and backwardly, as shown in FIG. 1. HST 8 comprises a variable displacement hydraulic pump 11 including a movable swash plate 13 and a fixed displacement hydraulic motor 21. Hydraulic pump 11 and motor 21 are fluidly connected with each other through a closed circuit comprising a first oil passage 93 and a second oil passage 94. Second oil passage 94 is hydraulically pressurized higher than first oil passage 93 during backward traveling. A charge oil passage 82 is provided for supplying the closed circuit with oil. Between charge oil passage 82 and each of first and second oil passages 93 and 94 is interposed a check valve 83. Check valve 83 provided for second oil passage 94 is bypassed by a drain oil passage including an orifice 116. Even if hydraulic pump 11 is inaccurately located in neutral, that is, swash plate 13 is slightly moved from its proper neutral position into its backward traveling range, increased pressure oil is drained from second oil passage 94 through orifice 116 so as to keep the hydraulic pressure in second oil passage 94 at its neutral level, thereby preventing a vehicle from jerking caused by unexpected driving of hydraulic motor 21.
Thus, movable swash plate 13 is provided in its movable range with a dead zone expanded from its proper neutral position into its backward traveling range for keeping hydraulic motor 21 neutral. For attaching the transaxle apparatus having HST 8 shown in FIG. 1 onto a vehicle, swash plate 13 is only required for sufficiently interlocking with a manual speed control tool such as a lever or a pedal to be positioned within the (lead zone while the speed control tool is set at its neutral position, thereby easing an adjustment job before the releasing of the vehicle equipped with the transaxle apparatus.
However, actually, the operating oil drained from second oil passage 94 through the drain oil passage is not returned directly into an oil sump 9 but is returned into charge oil passage 82. Hence, the heated oil drained from second oil passage 94 is absorbed through check valve 83 into hydraulically depressed first oil passage 93 while being insufficiently cooled, thereby increasing the temperature of operating oil circulated between first and second oil passages 93 and 94 and reducing the displacement efficiency of HST 8.
Furthermore, if hydraulic pump 11 and motor 21 are mounted onto a center section forming therein first and second oil passages 93 and 94 and a relief valve device including orifice 116 for expanding the neutral zone of HST 8 is mounted onto the center section so as to be connected to at least one of first and second oil passages 93 and 94, the center section requires additional manufacturing process for mounting the relief valve device thereon and connecting it to the oil passage, thereby increasing the manufacturing costs.